It's a tall order to take on a three mon core with just one poke, but Demantoid's answer actually does pretty well. Not having a guaranteed KO against Hippowdon when it's the usual lead (and when greninja is so frail) makes me nervous though.
Wow in my opinion aniravjain is definitely the best with his mega blastoise cresselia combo. That mega blastoise ohko's everything except for multiscale dragonite which is easily exploited by that rocky helmet cresselia. Amazing how that cresselia can set up tr and lunar dance to mega blastoise so it can ohko the remaining and you have yourself a fully healthy mega blastoise IN trick room AFTER beating those three pokemon!
I think Aegi and Thund-I work the best against this core from everything I didn't nom lol. Aegi just takes whatever from these Pokes and does a lot back, OHKOing stuff if WP has been triggered which is often the case. I think a bulkier Thundy with Taunt would be better, at least for this core. Taunt from a flying type completely shuts down most Hippo, and then you chip it with HP Ice. More bulk would help get off two T-Waves on DNite(cuz first is probably getting healed with a Lum Berry,) and the loss in power doesn't matter if you can still deal with Hippo(in fact, Taunt is sometimes better since specially defensive Hippos take a Grass Knot and then Toxic or set Rocks.)
EDIT: Don't let this thread die before I get a single vote lol!